Goto Admin panel -> Settings -> Languages Settings

First use Search for strings: _FORUM RSS FEED to see if the string is already present. If it is just edit it. If not do the step below.

Click "Add a new language key"

A pop up opens. Do the following:

New language key name: _FORUM RSS FEED
Category: choose one or create new one
Language string parameters: leave blank
String texts for available languages: FORUM RSS FEED

Click Save changes

Regards,

Harvliet